1. Field of the Invention
This invention relates to cathode structures for cathode ray tubes, and to a method for producing them, and more particularly relates to cathode structures produced from laminates of self-supporting layers wherein the bottom layer composition is optimized for bonding to the substrate, while the top layer composition is optimized for electron emissive properties.
2. Prior Art
Cathode structures for cathode ray tubes desirably exhibit uniform electron emissions over an extended lfe cycle and under a variety of operating conditions. In addition, such cathode structures must be manufactured at the lowest possible cost. Because of such stringent requirements, particularly reliability and cost, there is great reluctance on the part of high volume manufacturers of cathode ray tubes to introduce new cathode structures or methods. Nevertheless, presently used cathode structures and methods exhibit limitations sufficiently troublesome to justify continuing investigations of alternate structures and methods.
These limitations include poor adherence of the emissive layer to its substrate, non-uniformity of emissions, and variations in the cathode-to-grid spacings (K-G.sub.1 spacings) of the electron gun, resulting in out-of-specification values for cut-off voltages. Both non-uniformity of emissions and variations in the K-G.sub.1 spacings can result from non-uniformity in the thickness of the emissive layer. Particularly in the case of the sprayed coatings widely in use today, such non-uniform thicknesses occur not only from one cathode coating to another, but also within a single cathode coating. In addition, sprayed coatings tend to lack not only the thickness uniformity, but also the degree of surface smoothness of coatings produced by other techniques, such as casting a film of the potentially emissive material in an organic matrix. See. U.S. Pat. Nos. 2,974,364; 2,986,671; and 3,223,569, assigned to the present Assignee. Such variations in thickness and surface smoothness can lead to variations in quality of the spot produced from the impingement of the electron beam on the phosphor screen.
Adherence problems arise, particularly during operation near the high end of the normal temperature range, and can appear as lifting, flaking, or blistering of the emissive coating. Such adherence problems may be due in part to the dissimilarity of the substrate material, usually a nickel alloy, from the potentially emissive material, usually a mixture of alkaline earth carbonates. One approach to this problem has been the incorporation of some nickel into the emissive coating. See, for example, U.S. Pat. No. 3,879,830, assigned to the present Assignee. Unfortunately, such incorporation leads to a decrease in the emissivity of the coating. Another approach has been to form a separate layer of nickel or nickel-containing material between the substrate and the emissive layer, such as by spraying, brushing, plating, cladding, etc. See U.S. Pat. Nos. 3,327,158; 4,129,801; 4,114,243; 4,069,436; 4,053,807; 3,393,090; 3,374,385; 2,996,795 and 2,913,812. Such an approach obviously adds to the complexity of any manufacturing operation, particularly when the nickel undercoat requires further processing (e.g., sintering) prior to application of the emissive layer.
Accordingly, objectives of the present invention include: providing a cathode structure for cathode ray tubes which exhibits good adherence between the emissive layer and its supporting substrate; providing an emissive layer with optimum emissivity; providing an emissive layer which exhibits both uniformity of thickness and surface smoothness; and providing a method for producing such structures simply and reliably.